Method of determining characteristics of a tire from stresses

ABSTRACT

Method of determining at least one of the characteristics selected from: the three components of a resultant of forces which are exerted by the road on the contact area of a tire and the self-alignment torque generated by the tire, in which the said characteristic is derived from at least one measurement of the shear stresses at two fixed points in space, which are each situated in one of the beads.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/EP02/08620 filed Aug. 2, 2002, which claims priority of FrenchApplication No. FR01/10565 filed Aug. 6, 2001. The priority of theInternational Application is claimed under 35 U.S.C. § 120, and thepriority of the French Application is claimed under 35 U.S.C. § 119. TheInternational Application was published in French, but not in English,as WO 03014687.

BACKGROUND OF THE INVENTION

The present invention relates to vehicles, and to measurement of theforces which are exerted by the road on the tires of vehicles.

The present invention also relates to the various electronic assistancedevices used, for example, for antilock control of the brakes of avehicle or antiskid control of the drive wheels, control of thetrajectory of a vehicle or other forms of control or monitoring, forinstance the pressure of the tires.

In order to control the handling of a vehicle, attempts have been madeto determine certain rolling parameters. For instance, in order toreduce the longitudinal slip of the wheels, slip limitation systems(A.B.S., A.S.R.) have been developed which are capable of modulating thetorque transmitted to the wheel by the engine or the brake, as afunction of the slip derived from the variations in speed of rotation ofeach wheel. It has also been proposed to measure the torsion(circumferential angular deformation) of the sidewalls of a tire, inorder to determine the variations in the torque transmitted to the road.This measurement, which is more direct than derivation from thevariation in the speed of rotation, can allow more refined control ofthe slip limitation systems.

Systems (such as E.S.P.) are also known which influence the brakes orthe drive power applied to the wheels, in order to ensure that thetrajectory desired by the driver is actually followed by the vehicle. Inorder to do this, the yaw velocity (velocity of rotation of the vehicleabout a vertical axis), the rolling speed, the transverse accelerationof the vehicle and the angular position which the driver applies to thesteering wheel, are generally measured simultaneously.

SUMMARY OF THE INVENTION

The invention starts from the observation that all the forces exerted bythe road on the vehicle are transmitted via the wheels. It is thebalance of these forces which dictates the accelerations experienced bythe vehicle. Therefore, determining all these forces could make itpossible to dispense with the various sensors mentioned above, or tocomplement them in order to provide more complete information.

The method of the invention is based on recognition of the fact that theforces acting between the tread of the tire and the road cause asubstantial and reproducible modification of the stresses in the bead.These stresses, if one manages to measure them individually duringrotation of the tire in real time, can make it possible to know at eachinstant the direction and magnitude of the forces acting on the tire, aswell as the sign and the magnitude of the self-alignment torque exertedby the tire.

Owing to its design and its mode of operation, the deformations and theinternal stresses generated in the tire when it is constrained depend onits inflation pressure. The inflation pressure is therefore one of theparameters of the method proposed here. This pressure may be knownthrough a specific measurement means which is independent of themeasurements taken in the context of this invention, an example of sucha means being a pressure sensor. This pressure may also proceed fromspecific processing of the measurement of the stresses.

Under actual conditions of use, the tire is frequently subjected tovariations in the camber angle. This leads to a modification of thedeformations of the tire and of the distribution of the stresses in thebead. The camber is therefore one of the parameters of the methodproposed here. The camber may be known through a specific measurementmeans which is independent of the measurements taken in the context ofthis invention, an example of such a means being a camber angle sensor.This camber may also proceed from specific processing of the measurementof the stresses in the beads.

The invention proposes a method of determining at least one of thecharacteristics selected from: the three components of a resultant offorces which are exerted by the road on the contact area of a tire, theself-alignment torque generated by the tire, the camber and thepressure, the characteristic being derived from at least one measurementof the stresses at at least three fixed points in space, which aresituated in one of the beads. Preferably, the said at least three fixedpoints in space are such that:

one of the points corresponds to the azimuth of the center of thecontact area or the azimuth of the point opposite to the contact area;

the other two points are symmetrical with respect to a vertical planepassing through the center of the contact area.

The rest of the description presents the case in which shear stressesare measured. This approach, however, should not be considered asimplying limitation, and other measurements of stresses, for exampleflexural or compressive stresses observed at the same positions, mayalso make it possible to determine the same characteristics.

In a preferred embodiment, the invention proposes to estimate the shearstress in the bead in the circumferential direction. The measurement ofthe shear stresses may, for example, be carried out in the zone wherethe carcass is anchored in the bead, preferably in a rubber componentwhose Young's modulus is fairly high, preferably more than 5 MPa at 10%strain. For example, a sensor is installed in the rubber constituentsuperimposed on the bead wire although this is only a particular caseamong many possible configurations, depending on the design of thetires.

BRIEF DESCRIPTION OF THE DRAWINGS

The rest of the description explains the invention in more detail withthe aid of the appended figures, in which:

FIG. 1 is a perspective of a tire on which the conventions useful forunderstanding the invention are defined; the circumferential shearstress corresponds to the shear between the radial direction (denoted rin the figure) and the circumferential direction (denoted “c” in thefigure). This shear stress will be denoted σ_(rs)

FIG. 2 a shows the effect of the vertical component Fz on the stressσ_(rs) for Point A of the tire of FIG. 1, where:

-   -   a. the solid curve corresponds to a vertical load of 400 daN,    -   b. the dotted curve corresponds to a vertical load of 500 daN,        and    -   c. the dotted and dashed curve corresponds to a vertical load of        300 daN;

FIG. 2 b shows the effect of the vertical component Fz on the stressσ_(rs) for Point B of the tire of FIG. 1, the curves representing thesame loads as in FIG. 2 a;

FIG. 3 a shows the effect of the component Fx on the stress σ_(rs) forPoint A shown in the tire of FIG. 1, where:

-   -   a. the solid curve corresponds to a vertical load of 400 daN and        an absence of any force Fx,    -   b. the dotted curve corresponds to a vertical load of 400 daN        and a force Fx of −400 daN (braking), and    -   c. the dotted and dashed curve corresponds to a vertical load of        400 daN and a force Fx of 400 daN (driving);

FIG. 3 b shows the effect of the component Fx on the stress σ_(rs) forPoint B of the tire shown in FIG. 1;

FIGS. 4 a and 4 b show the effect of the component Fy on the stressσ_(rs):

-   -   a. where the solid curve corresponds to a vertical load of 400        daN without any force Fy,    -   b. where the dotted curve corresponds to a vertical load of 400        daN with a force Fy of 280 daN, and    -   c. where the dotted and dashed curve corresponds to a vertical        load of 400 daN with a force Fy of −280 daN;

FIG. 4 b shows the effect of the component Fy on the stress σ_(rs) forPoint B of the tire shown in FIG. 1;

FIG. 5 shows the deformation of the tire when a camber angle is applied;

FIG. 6 a shows the effect of the camber on the shear stress signals forPoint A of the tire as shown in FIG. 1, where:

-   -   a. the solid curve corresponds to a vertical load of 400 daN        without any forces Fx and Fy, and to a zero camber angle,    -   b. the dotted curve corresponds to a vertical load of 400 daN        with a camber angle of 2°, and    -   c. the dotted and dashed curve corresponds to a vertical load of        400 daN with a camber angle of 4°;

FIG. 6 b shows the effect of the camber on the shear stress signals forPoint B of the tire shown in FIG. 1;

FIG. 7 shows the architecture of a neural network;

FIG. 8 shows examples of transfer functions;

FIG. 9 a shows an example of an architecture allowing the inflationpressure of the tire to be taken into account if it varies using adedicated pressure measuring device;

FIG. 9 b shows an example of an architecture allowing the inflationpressure of the tire to be taken into account if it varies using thestress sensor to derive pressure;

FIG. 10 shows an example of a shear stress sensor consisting of aninverted T-shaped test body equipped with two stress gauges;

FIG. 11 shows a perspective section view of an example of theinstallation of the sensor presented in FIG. 7 in the region of the beadof the tire;

FIG. 12 shows in section view in the meridian plane an example of theinstallation of the sensor presented in FIG. 7 in the region of the beadof the tire;

FIG. 13 shows the raw and filtered time signal;

FIG. 14 shows the identification of passage through the contact area onthe basis of the time signal;

FIG. 15 shows an example of operation with one sensor and one model;

FIG. 16 shows an example of operation with three sensors and one model;

FIG. 17 shows an example of operation with three sensors and two modelswhere:

-   -   a. the positions indicated by solid lines represent the azimuths        at which the measurements for use as the input for model 1 are        to be taken,    -   b. the positions indicated by dotted lines represent the        azimuths at which the measurements for use as the input for        model 2 are to be taken, and    -   c. C1, C2 and C3 represent the azimuthal positions of the        sensors on the bead of a tire.

DETAILED DESCRIPTION OF THE INVENTION

The method described here relies on the fact that each force applied tothe tire in the contact area causes a modification of the shear stressin the bead. The case of an inflated tire mounted on its wheel will beconsidered, on whose first bead a point A is identified at the level ofthe bead. On the second bead, at the same azimuth as A and on the sameradius, a point B is selected. In the absence of any forces beingapplied to the tire, the shear stress is constant as a function of theangle of rotation of the tire-wheel assembly, and it corresponds to theresidual inflation stress.

When the tire is subjected to forces, the following effects are observedfor each of the components of the said forces:

The vertical component (denoted by Fz here) presses the tire onto theground. By creating a contact area, it leads to a variation of the shearstress at point A when the fitted assembly is in rotation. FIGS. 2 a and2 b indicate the shear stress, respectively at point A and at point B,as a function of the azimuth where they lie. The tire belt is connectedto the bead via the sidewalls. The increase of the applied verticalcomponent leads to a vertical displacement of the wheel with respect tothe tire belt. The sidewalls then shear the bead in opposite directionsat the entry and exit of the contact area. It also worth noting that theshear stress remains zero at the azimuth of the center of the contactarea, as well as at the point opposite to the center of the contactarea.

The horizontal component in the rolling direction (denoted by Fx here)is created by a driving or braking torque applied to the wheel. Thisentails a rotation of the wheel with respect to the tire belt. Thesidewalls are carried along by the belt and shear the bead over all theazimuths. FIGS. 3 a and 3 b illustrate the effects of the component Fxof the applied forces by indicating the shear stress at points A and B,as a function of the azimuth where they lie. When a positive force Fx isapplied (driving torque), the shear stress, as it is defined, decreasesover all the azimuths on both beads. When a negative force Fx is applied(braking torque), the shear stress increases over all the azimuths onboth beads.

The horizontal component in the transverse direction (denoted by Fyhere) mainly causes differentiation between the two beads. FIGS. 4 a and4 b illustrate the effects of this type of constraint by indicating theshear stress at points A and B, as a function of the azimuth where theylie. In the case of a constraint with positive Fy, one of the beadsshows an increase in the shear stress on the entry side of the contactarea and a decrease on the exit side. The other bead shows a decrease inthe shear stress on the entry side of the contact area and an increaseon the exit side. In contrast to when a load is applied, a variation ofthe shear stress in opposite directions at the points with azimuths 180°and 0° is observed on the two beads when a force Fy is applied.

The self-alignment torque N (moment about the vertical axis) is not,strictly speaking, a force which is imposed. Rather, it is a consequenceof the way in which the components Fx, Fy and Fz are applied in thecontact area. If the point of application of the resultant, whosecomponents are Fx, Fy and Fz, is not the center of the contact area,this resultant generates a moment about Oz, which is referred to as theself-alignment torque. The existence of this moment principally entailsa rotation of the contact area about Oz. The consequence of this effectis, for example, an increase in the shear stress in one bead at theazimuth of the center of the contact area and a decrease in the shearstress in the other bead at the same azimuth, with respect to asituation with zero self-alignment torque.

In the event that a camber angle is applied to the tire, the behavior ofthe two beads is different. Simplistically, everything happens as if onebead were carrying more load than the other. FIG. 5 illustrates thisbehaviour by comparing a cross section of the part of the tire in thecontact area without any camber and with a camber y. This also resultsin a slight lateral displacement of the contact area, which entails athrust in the Y direction. FIGS. 6 a and 6 b show the change of theshear stresses in the two beads. On the overloaded bead (point A), thechange is similar to that of a load increase. On the other bead (pointB), a change is seen which is compatible with a decrease in the loadbeing supported. Given that the changing of the signals is odd inrelation to the beads and odd in relation to the azimuth, just like theeffect of Fy, it is readily possible to distinguish an effect of thecamber from an effect of the Fx, Fz or N type. FIGS. 4 and 6(a and b)furthermore show that the effects of Fy and of the camber angle differ.It is therefore possible to establish an unambiguous relationshipbetween the stress signals and the camber. It is then possible toestimate the value of the camber angle at which the tire is working,with the aid of the measurements of stresses in the bead.

The apparent rigidity of a tire originates both from its pneumaticbehaviour (from its inflation pressure) and from its structural rigidity(rigidity of its architecture). The measured stress signals themselvesalso contain a pneumatic component and a structural component. Forexample, the stress signals of a tire inflated to 2 bar and loaded with400 daN along Z are not identical to those delivered by the same tire at2.5 bar and loaded with 500 daN. This difference corresponds to thestructural contribution, and can make it possible to estimate theinflation pressure of the tire.

In the event that the inflation pressure varies, the relationships whichlink the applied forces and the stress signals are quantitativelymodified, but without their nature being changed. The stresses in thebeads are influenced by the pressure and by the load; they are made upof a contribution due to the “pneumatic” behaviour (that is to saydependent on the inflation pressure) and another contribution due to thestructural behaviour (that is to say of the constituent materials of thetire and their arrangement), which does not change when the pressurechanges, so that information about the pressure can be obtained.

The method may thus be explained firstly in the case of an inflationpressure which is assumed to be constant, for the sake of simplicity.Likewise, it will be considered below that the camber is constant andzero, in order to make the explanation clearer, and only the mostinteresting cases concerning this parameter will be mentioned.

Before continuing with the detailed description of several examples, inwhich measurement of the stresses is always carried out at at least twofixed points in space, it should be noted that there is at least onecase in which a stress measurement in a single bead makes it possible toestimate one of the components of a resultant of forces. Indeed, as canbe seen in FIGS. 2 a, 2 b, 3 a, 3 b, 4 a and 4 b, the components Fy orFz applied in the contact area have no effect on the shear stressmeasured at the azimuth opposite to the contact area (azimuth 0°). Themeasurement of the shear stress at this point therefore makes itpossible, by itself, to estimate the component Fx of the forces whichare applied in the contact area.

When a constraint which mixes components Fx, Fy and Fz is applied, asuperposition of the aforementioned effects on the circumferential shearstress is observed. One of the advantages of the proposed method is thatit makes it possible to separate the contributions of each component ofthe applied constraint, so as to make it possible to estimate each ofthese components.

The approach which is used relies partly on significant paritycharacteristics, which correspond to the natural symmetries of the tire,in order to carry out this separation.

The azimuth θ will be defined as the angle at which the circumferentialshear stress of the beads is analysed. The origin of the azimuth istaken on the opposite side from the center of the contact area. Thecenter of the contact area therefore has the azimuth 180°.

The stress signal as a function of the azimuth, s(θ), can then bedivided into two signals s_(p) (θ) and S_(i) (θ), which are such that:s(θ)=s _(p)(θ)+s _(i)(θ)s _(i)(180+θ)=−s _(i)(180−θ)s _(p)(180+θ)=s _(p)(180−θ)s_(i) is referred to as the odd part of s, and s_(p) is referred to asthe even part of s.

Likewise, let s¹(θ) and s²(θ) be the signals associated with measurementof the circumferential shear stress on each of the sides of the tire.The following are defined: $\begin{matrix}{{s^{p}(\theta)} = \frac{{s^{1}(\theta)} + {s^{2}(\theta)}}{2}} \\{{s^{i}(\theta)} = \frac{{s^{1}(\theta)} - {s^{2}(\theta)}}{2}}\end{matrix}$

s^(p) is referred to as the bead-related even part and s^(i) is referredto as the bead-related odd part.

It should be noted that this division by parity according to the beadsmay equally well be applied to s_(i) and s_(p). Four signals s_(i) ^(i)s_(i) ^(p) s_(p) ^(i) s_(p) ^(p) are then obtained on the basis of ameasurement carried out on each bead.

The forces Fx, Fy, Fz and the self-alignment torque N are, owing totheir orientations, linked with certain symmetries. In particular, thisprinciple can be used to decouple the effects of the force components onthe tire.

Hence, according to the observations (FIGS. 2 a, 2 b, 3 a, 3 b, 4 a and4 b), the signal:

-   -   s_(p) ^(p) is mainly linked with the force Fx.    -   s_(i) ^(i) is mainly linked with the force Fy    -   s_(i) ^(p) is mainly linked with the force Fz

The symmetries which apply furthermore make it possible to confirm thatthe signal s_(p) ^(i) is principally linked with the self-alignmenttorque N.

By virtue of these observations, the method explained here proposes tocarry out measurements of the circumferential shear stress in the beadon at least one side of the tire. Thanks to mathematical operations(linear or non-linear combinations of the measurements carried out atthe various azimuths), these measurements make it possible to estimatethe values of the signals s_(i) ^(p) s_(p) ^(i) s_(p) ^(p) and s_(i)^(i) at certain azimuths, and thereby to provide an evaluation of thecomponents of the applied force.

With a view to clarifying the procedure, some examples in which themethod is used, but which are not exhaustive and in no way limit theusable configurations to those listed here, are presented here.

The case in which the measurements are carried out on only one bead willbe considered.

EXAMPLE 1

The intention is to estimate the components of the forces which areapplied in the contact area, and the self-alignment torque, on the basisof measurements of the circumferential shear stress measured in one beadof the tire, at three azimuths. The measurement azimuths are selected inthe following way:

One of the azimuths corresponds to the middle of the contact area or theazimuth of the point opposite to the contact area (azimuth 180°). LetV_(c) be the value measured at this point.

The other two azimuths are symmetrical with respect to the azimuth ofthe center of the contact area. (180°+α° and 180°−α°). Let V₁ and V₂ bethe values measured at these points.

According to the observations above:

V₂−V₁ makes it possible to estimate the imbalance between the entry ofthe contact area and the exit. This value will be principally linkedwith the component Fz. An estimate of Fz is given by f_(z)(r₂V₂−r₁V₁),where r₁ and r₂ are positive real coefficients and f_(z) is a monotoniccontinuous function.

V_(c)−(V₁+V₂) makes it possible to estimate the difference betweenpassage through the contact area and outside the contact area. Theresult here is principally linked with Fy. An estimate of Fy is given byf_(y)(s_(c)V_(c)−(s₁V₁+s₂V₂)), where S₁, s₂ and s_(c) are positive realcoefficients and f_(y) is a monotonic continuous function.

V_(c)+V₁+V₂ gives an indication of the overall shear of the bead. Thisvalue will be principally linked with the component Fx of the appliedforce. An estimate of Fx is given by f_(x)(u_(c)V_(c)+u₁V₁+u₂V₂), whereu₁, u₂ and u_(c) are positive real coefficients and f_(x) is a monotoniccontinuous function.

In this example four components (Fx, Fy, Fz and N) are estimated on thebasis of three measurements of the circumferential shear stress. Indeed,there are cases in which the self-alignment torque is dependent directlyand only on the components Fx, Fy and Fz. It can then be estimated aswell. In the event that the self-alignment torque depends on otherparameters, it is necessary to measure the circumferential shear stressin the bead at a greater number of azimuths, in order to estimate thesaid four force components correctly.

EXAMPLE 2

The intention is to estimate the components of the forces which areapplied in the contact area, and the self-alignment torque, on the basisof measurements of the circumferential shear stress in the bead on oneside of the tire, measured at five azimuths. The measurement azimuthsare selected in the following way:

-   -   One of the azimuths corresponds to the middle of the contact        area (azimuth 180°) or to the opposite side from the contact        area (azimuth 0°). Let V_(c) be the value measured at this        point.

Two other azimuths are symmetrical with respect to the azimuth of thecenter of the contact area. (180°+α° and 180°−α°). Let V₁ and V₂ be thevalues measured at these points.

The final two azimuths are symmetrical with respect to the azimuth ofthe center contact area. (180°+β° and 180°−β°). Let V₃ and V₄ be thevalues measured at these points.

Combinations which are of the same nature as, but a little more complexthan, those explained in Example 1 make it possible to determine thecomponents Fx, Fy, Fz and N in this case, including cases in which theself-alignment torque is dependent not only on the components Fx, Fy andFz.

The case in which the measurements are carried out on both beads willnow be considered.

EXAMPLE 3

The intention is to estimate the components of the forces which areapplied in the contact area, and the self-alignment torque, on the basisof measurements of the circumferential shear stress of both beads of thetire, measured at two azimuths on each bead. The measurement azimuthsare selected symmetrically with respect to the azimuth of the center ofthe contact area (180°+α° and 180°−α°). So that Fx can be estimated, αmust not be equal to 0° or 180°. Let V₁ ¹ and V₂ ¹ be the valuesmeasured at these azimuths on the first bead, and V₁ ² and V₂ ² thevalues measured at these azimuths on the second bead.

Thanks to these four values, it is possible to determine the componentsby using decomposition according to the azimuth-related and bead-relatedparities:

V₁ ¹+V₁ ²+V₂ ¹+V₂ ² gives the azimuth-related and bead-related evencomponent. This combination is therefore directly linked with Fx. Anestimate of Fx is given by f_(x)(a₁V₁ ¹+a₂V₂ ¹+b₁V₁ ²+b₂V₂ ²), where a₁,a₂, b₁ and b₂ are positive real coefficients and f_(x) is a monotoniccontinuous function.

V₁ ¹+V₁ ²−(V₂ ¹+V₂ ²) gives the azimuth-related odd and bead-relatedeven component. This combination is therefore directly linked with Fz.An estimate of Fz is by f_(z)(c₁V₁ ¹−c₂V₂ ¹+d₁V₁ ²−d₂V₂ ²), where c₁,c₂, d₁ and d₂ are positive real coefficients and f_(z) is a monotoniccontinuous function.

V₁ ¹−V₁ ²+(V₂ ¹−V₂ ²) gives the azimuth-related even and bead-relatedodd component. This combination is therefore directly linked with N. Anestimate of N is given by f_(n)(e₁V₁ ¹+e₂V₂ ¹−f₁V₁ ²−f₂V₂ ²), where e₁,e_(2, f) ₁ and f₂ are positive real coefficients and f_(n) is amonotonic continuous function.

V₁ ¹−V₁ ²−(V₂ ¹−V₂ ²) gives the azimuth-related odd and bead-related oddcomponent. This combination is therefore directly linked with Fy. Anestimate of Fy is given by the real coefficients f_(y)(g₁V₁ ¹−g₂V₂¹−h₁V₁ ²+h₂V₂ ²), where g₁, g₂, h₁ and h₂ are positive real coefficientsand f_(y) is a monotonic continuous function.

This type of arrangement makes maximum use of the symmetries of thetire, and very good precision may be expected when reconstructing thecomponents of the constraint applied in the contact area.

EXAMPLE 4

The intention is to estimate the components of the forces which areapplied in the contact area, and the self-alignment torque, on the basisof measurements of the circumferential shear stress in the bead on bothsides of the tire, measured at three azimuths on each bead. Themeasurement azimuths are selected in the following way:

Two azimuths selected symmetrically with respect to the azimuth of thecenter of the contact area (180°+α° and 180°−α°). Let V₁ ¹ and V₂ ¹ bethe values measured at these azimuths on the first bead, and V₁ ² and V₂² the values measured at these azimuths on the second bead.

One azimuth corresponding to the center of the contact area. Let V_(c) ¹and V_(c) ² be the values measured at these azimuths. The azimuth whichcorresponds to the opposite side from the contact area can be usedequivalently.

The processing is similar to that in Example 3. The values V_(c) ¹ andV_(c) ² allow a certain redundancy of the information, but above allbetter estimation of the component Fx.

The information about Fx is obtained with the aid of V_(c) ¹ and V_(c)², and the information about Fz, Fy and N is obtained using V₁ ¹, V₁ ²,V₂ ¹ and V₂ ². An additional possibility for decoupling the variouscontributions is hence used.

EXAMPLE 5

The intention is to estimate the components of the forces which areapplied in the contact area, and the self-alignment torque, on the basisof measurements of the circumferential shear stress in the bead on bothsides of the tire, measured at four azimuths on each bead. Themeasurement azimuths are selected in the following way:

Two azimuths selected symmetrically with respect to the azimuth of thecenter of the contact area (180°+α° and 180°−α°). Let V₁ ¹ and V₂ ¹ bethe values at measured these azimuths on the first bead, and V₁ ² and V₂² the values measured at these azimuths on the second bead.

One azimuth selected with respect to the azimuth of the center of thecontact area (180°+β). β is not equal to α. Let V₃ ¹ be the valuemeasured at this azimuth on the first bead, and V₃ ² the value measuredat this azimuth on the second bead.

One azimuth corresponding to the center of the contact area. Let V_(c) ¹and V_(c) ² be the values measured at these azimuths. The azimuth whichcorresponds to the opposite side from the contact area can be usedequivalently.

In this case, processing similar to cases 3 and 4 is applicable, withmore robustness in view of the redundancy of the information. Inaddition to estimating the force components and the torque N, however,the stress measurements proposed here furthermore make it possible toprovide an estimate of the camber angle, in the event that the latter isliable to vary. Indeed, the difficulty in this case involves determiningthe part of the two azimuth-related odd and bead-related oddcontributions constituted by the component Fy and the camber angle.

The provision of measurements at two different angles with respect tothe center of the contact area makes it possible to evaluate the slopeof the signals as a function of the azimuth, and to discriminate an Fyeffect from a camber effect. Carrying out the shear stress measurementsin both beads allows the estimations to become much more robust inrelation to the camber variation, and also allows to estimate the camberangle.

The linear combinations taken by way of example above are veryrudimentary, and only allow the principal effects to be taken intoaccount. With a view to refining the estimations of the components ofthe forces and to taking the non-linear behaviour of the tire intoaccount, the described method resorts to more sophisticated transferfunctions for relating the measurements to the estimates of the forces.Any interpolation function making it possible to establish a linkbetween the measured quantities and the values of the selectedcharacteristic or characteristics may be used in this context. Thus thecoefficients of the interpolation function may be determined with theuse of a training base (see below).

Although all the examples listed here use measurement azimuths which areselected so as to take maximum advantage of the symmetries of the tireand to facilitate reconstruction, the selection of the position of theazimuths at which the values are measured is free (symmetry of theazimuths is not obligatory per se), because any combination of asufficient number of measurements makes it possible to estimate thecomponents of the applied constraint. It is possible, in this case, tolook directly for the functions giving the components Fx, Fy, Fz and Nas a function of the measurements of the circumferential shear stress inthe bead on one side or on both sides, at known azimuths. Thedetermination of the transfer functions is no longer based necessarilyon analysis of the mechanics of the tire, but rather on the response ofthe tire, in terms of circumferential shear stress in the bead on oneside or on both sides, to the forces which it experiences.

Whether the measurement azimuths are selected thanks to a physicalanalysis or decided more arbitrarily, neural networks seem highlysuitable for establishing a transfer function between the measurementswhich are carried out and the components of the forces Fx, Fy, Fz and N.If appropriate, the camber angle may also be one of the quantities to beestimated, and it may appear at the output of the transfer function forestablishing the simplest applicable schemes, the use of networks havingone layer of hidden neurons and one layer of output neurons may beadopted as the interpolation function for establishing a link betweenthe measured quantities and the values of the components of the appliedconstraint. These hidden neurons use a sigmoid transfer function. Theoutput neurons, for their part, use a linear transfer function (FIG. 7).The parsimony property of this type of network, when used as anapproximator, is very beneficial here. It is possible to use one networkper component to be estimated, or a network that makes it possible toestimate all the components thanks to a plurality of outputs.

If the measurement azimuths have been selected so as to take advantageof the symmetries or physical observations, it may be beneficial to makelinear combinations of the quantities before input into the network. Inthis case, a principal component analysis will make it possible todetermine the coefficients of these combinations expediently, and willsimplify the required neural network. The architecture described in FIG.8 is obtained, which shows examples of transfer functions for which theinput linear combinations are optional. It is possible to use a networkwith a plurality of outputs, or a plurality of networks with one output,or any other combination. The possible output quantities (Fx, Fy, Fz, N,P and γ) are indicated, but they are of course optional.

Specifically, the operation is carried out as follows:

The first step, after having determined the measurement azimuths,consists in collecting the values of the circumferential shear stress inthe bead on at least one side, during varied constraints of the tirewhich are selected so as to cover the full range in which evaluation ofthe selected characteristic or characteristics will be permitted innormal use. The selected constraints also need to involve all thecouplings liable to be encountered during normal use. The set ofmeasured values and the associated selected characteristic orcharacteristics (obtained by another measurement means) constitute thetraining base. Of course, in the event that the camber is subsequentlyliable to vary, it is desirable to incorporate variations of the camberangle which are representative of the future range of use into thetraining base.

The second step consists in carrying out the training of the weightingsof the network (or, more generally, carrying out the determination ofthe coefficients of an interpolation function) on the base formed inthis way. At the end of this phase, the transfer functions are obtained.

A third step consists in testing the transfer functions by comparing theestimates of the selected characteristic or characteristics with thevalues indicated by another measurement means.

Besides neural networks, it is possible to use polynomial functions, forexample.

In the most realistic case, in which the inflation pressure of the tireis liable to change in the course of time, it may be necessary to takethe pressure variations into account, depending on the precision desiredfor the measurement of the components in question.

A first procedure consists in correcting the estimated forces at theoutput of the transfer function as a function of the pressure. It isthus possible to carry out a first-order correction. Indeed, let therebe a constraint applied to the tire in the event of a transfer functionwhich does not take the pressure into account. If the pressure is doublethe reference pressure (at which the transfer function was established),the transfer function will see about two times less measured stresses asinput than for the reference pressure. It will therefore evaluate forcesthat are two times weaker than the forces actually being applied. Theestimated forces should be multiplied by two.

The most precise approach, however, consists in introducing the pressureas a parameter in the transfer functions. This involves:

Carrying out the training of the transfer function or functions on atraining base containing cases in which the tire is constrained undervarious conditions of inflation pressure covering the desired range ofoperation.

Having at one's disposal a measurement or an estimate of the inflationpressure, by measurement of the stresses themselves or by anotherancillary device.

Without implying any limitation, two ways of knowing the pressure willbe described below. The first consists in using a pressure measurementgiven by a pressure sensor which is different from the stress sensors.The measured pressure value is then supplied to the transfer function orfunctions, in addition to the values of the stresses at the azimuths.FIG. 9 a illustrates a schematic of the associated architecture.

The second approach consists in estimating the inflation pressure on thebasis of the stress measurements. Indeed, the stress signals have astructural component and a pneumatic component, which makes it possibleto obtain information about the inflation pressure by analysing them.This way of proceeding requires the determination of a transfer functionwhich takes the measurements of stress at the desired azimuths as itsinput, and which gives an estimate of the inflation pressure over theintended range of operation. The same methodology as that presentedabove is applicable:

Formation of a training base which mixes variations in the appliedforces and in the inflation pressure.

Determination of a Transfer Function by Training.

In practice, if the precision of a pressure determination performed asindicated above is deemed insufficient for a particular embodiment ofthe invention, it can be improved easily. Indeed, the change of thepressure in a tire is a phenomenon which is slow compared with therotation of the tire. The pressure estimates can therefore be averagedor filtered so as to keep only the low-frequency components. A goodestimate of the inflation pressure is then obtained. FIG. 7 bschematises the architecture which results from this approach. Besidesknowledge of the resultants of forces in question, the method thenprovides an estimate of the inflation pressure without any additionalsensor.

Naturally, many other variables (in addition to the measurements of thecircumferential shear stress in the bead) may be taken into accountaccording to the same principle, in order to improve the efficiency ofthis determination. Such is the case, for example, concerning thetemperature of the tire or the speed of rotation. Indeed, depending onthe type of sensor and the position of the measurement, it may be thatthe stress signals which are obtained depend slightly on the speed ofrotation of the tire. In order to improve the precision of theestimates, it may then be beneficial to add the speed of rotation as aninput parameter of the transfer function. Knowledge of the speed maythen come from a measurement carried out by another component installedon the vehicle or, for example, it may be extracted from the stresssignals themselves.

In general, the number of measurement points may be higher than theminimal configurations presented in the examples, and may permit aresult which is more precise or more reliable because of the redundancyof the available information.

An alternative way of increasing the precision or the robustness of themethod consists in using a multi-dimensional measurement instead of aone-dimensional measurement. For example, and without implying anylimitation, both a circumferential shear stress and a transverse shearstress may be used, the two quantities being measured preferably, butwithout implying any limitation, simultaneously by the sametwo-dimensional sensor at the same position.

The use of these two stresses makes it possible to provide aconfiguration, in which a single bead is equipped with sensor(s), whichis as robust in terms of performance and as precise as a configurationin which both beads are equipped. In particular, this configurationmakes it possible to measure the camber even though just one bead isequipped with sensor(s), which is not possible when using aone-dimensional sensor in a single bead.

In this case, the inputs of the transfer function consist of anassortment of measurements of one or the other or different types ofstresses at various azimuths. Apart from this difference, exactly thesame procedure is used for determining the transfer function. Thisapproach turns out to be very beneficial because, in terms of producingthe final product, it may be much simpler and less expensive to equiponly a single bead, even if the sensor itself is more expensive tomanufacture.

The measurement of the shear stress in the bead 1, on one side or onboth sides of the tire, may be performed in any manner, using a devicewhich is external to the tire or a device which is internal to the tire.By way of example, the use of one or more sensors 3 which are placed inthe tire in an anchoring zone 2 of the carcass, and which are thereforecarried along in rotation by the tire, will be described here formeasuring the circumferential shear stress in the bead 1.

This sensor or these sensors 3, integrated with the tire and locallymeasuring the circumferential shear stress of the bead or beads, mayemploy any physical measurement principle. They may, for example,consist in a test body 30 equipped with stress gauges 31, for exampleresistive gauges. Deformation of the test body leads to a modificationof the resistance of the stress gauges 31 bonded to its surface (FIGS.10, 11 and 12). Via a Wheatstone bridge, the two gauges 31 placed oneither side of the test body 30 then provide a signal which is stronglylinked with the circumferential shear stress. If it is active, thesensor 3 may be powered either by the vehicle, using wireless supply, orby a battery installed on the wheel or in the tire, or by any othermeans. Everything is also possible concerning the transmission of theinformation to the vehicle, by radio or other means. The sensor 3 per semust be capable of delivering information continuously, or with arefresh frequency which is fast enough in relation to the period ofrotation of the wheel.

One possible variant of the sensor 3 consists in using piezoelectricelements instead of the stress gauges 31. The piezoelectric elements areconnected to a charge amplifier, which makes it possible to deliver avoltage proportional to the flexure of the test body. This variant hasthe benefit of not only providing a measurement signal, but alsopotentially of supplying electrical energy to the circuits forprocessing and/or for transmitting the information.

For measuring the circumferential shear and transverse shear stresses,it is also possible to use a sensor of the type described in PatentApplication EP02/014144, placed in the bead. This Application describesa force sensor of the nail type, having a rigid stem intended to beconstrained by the force to be picked up, and a head which has anelement that is secured to the stem and is intended to be deformed orstressed when the stem is constrained. The nail-type sensor is arrangedat the same position as the sensor 3 shown in FIG. 11, the stem beingoriented substantially radially and towards the largest radii, oralternatively the stem being oriented substantially perpendicularly tothe orientation indicated above, and towards the internal cavity of thetire.

This approach, using a sensor 3 or the like integrated with the tire,has the advantage of making it possible to know the shear stress in thebead on one side or on both sides, and at all the azimuths of the tire,since a sensor 3, when being carried along by the tire, explores all theazimuths during a rotation of the wheel.

The fact that the method of reconstructing the components of the forcesis based on measuring the circumferential shear stress at certainazimuths entails the problem of locating the sensor 3 in order toextract the values at the correct azimuths.

The sensor 3 is interrogated, or delivers on its own a measurement, at aconstant and known frequency. It therefore delivers a time signal of thevariation in the local circumferential shear stress. A measured signalis presented in FIG. 10. On this time signal, it is easy to see thesignature of a wheel revolution which was observed previously (FIGS. 2a, 2 b, 3 a, 3 b, 4 a and 4 b). Further to the signature of each wheelrevolution, this signal contains noise. The first operation consists inreducing this noise by applying a low-pass filter, the cut-off frequencyof which may be linked with the speed of rotation of the wheel.

Several cases may then arise, depending on the available equipment:

-   -   If a measurement of the angular position of the wheel is        available, it is possible to know the instants at which the        sensor 3 passes through the measurement azimuth. Reading the        values measured at these instants provides the value of the        shear stress at the desired azimuths. This measurement of the        angular position of the wheel may, for example, be obtained by        counting the transitions of an ABS sensor for the speed of        rotation of the wheel.

If no external device is available to facilitate the location of thesensor 3, only the signal of the sensor itself can be used. Theinvention proposes to use the signal of the sensor, or of other sensors,if any, integrated with the tire, in order to estimate the angularposition of the wheel.

Each passage of the sensor 3 through the contact area has, as itssignature, a rapid variation of the shear stress in the bead, whichleads to a pronounced slope. By using this observation, it is possibleto find the instants at which the sensor 3 passes through the center ofthe contact area. The simplest method for carrying out this operationconsists in differentiating the filtered signal with respect to time, soas to ascertain the slope at each instant. The differentiated signalwhich is obtained has an extremum during passage through the center ofcontact area. It is then possible to carry out thresholding of thisdifferentiated signal and to look for the maxima among the valuesgreater than this threshold (“algorithm 1”—FIG. 14). This approach makesit possible to avoid detecting the maxima which do not correspond topassage through the contact area. The instants at which the extrema havebeen located correspond to the instants at which the sensor 3 passesthrough the center of the contact area.

The shape of the signal changes substantially as a function of theapplied forces. In real conditions, the thresholding may turn out to becomplicated, because the level of the threshold needs to be adaptedconstantly. Furthermore, under certain conditions, it may happen thatthe application of a threshold causes the detection of several extremaper wheel revolution. This situation is encountered when a large forceFy is applied. One possible approach, but not the only approach,consists in using the following algorithm:

Using by default the previously explained algorithm, referred to as‘algorithm 1’.

When a periodicity is detected, using the date of the last passagethrough the contact area, and an evaluation of the speed based on thelast passages, in order to predict the date t_(n) of the future passagethrough the contact area.

Defining a signal window [t_(n)−d;t_(n)+d] around t_(n) with the aid ofan uncertainty, d being less than half the period of the signal.

Carrying out the thresholding in this window in order to determine thetrue date T_(n) which corresponds to the approximation t_(n).

Performing a new iteration in order to detect the next revolution. Inthe event of an error (visibly false period, extremum found at the edgeof the window, etc.) repeating ‘algorithm 1’ in order to re-synchronisethe process.

Each time another passage through the contact area is determined,knowledge of the instants of the last passages (at least 3 passages)makes it possible to estimate the speed of rotation of the wheel and itsacceleration. Thanks to these estimates, it is possible to reconstructan evaluation of the azimuth at which the sensor 3 lies as a function oftime. It then becomes possible to extract the values at certain azimuthsfrom the measurements as a function of time.

As seen above, the estimate of the speed of rotation can be used as aninput of the transfer function, in order to improve the precision of theestimation of the force components over a large range of speeds.

Several options are then available for implementing the measurement.Indeed, determining the components of the forces requires measurementsat a plurality of azimuths.

A first approach consists in using only one sensor 3 in each bead forwhich measurements are intended to be obtained. At each passage througha required position, the value given by the sensor is taken into accountin order to refresh the measurement at the azimuth in question. Bymaking the assumption that the components of the forces vary slowly inrelation to the speed of rotation of the wheel, a single sensor thusmakes it possible to obtain the measurements at all the azimuthsnecessary for reconstruction of the forces. FIG. 15 presents this typeof operation with a model (transfer function) which requiresmeasurements at three azimuths (0°, 120° and 240°).

A second approach consists in providing a plurality of sensors 3 overthe circumference, so that, at least once per revolution, the sensorssimultaneously lie at the azimuths where a measurement is intended to becarried out. It is thus possible to obtain an image of the deformationof the tire at various azimuths at a given instant, which no longerrequires that the forces vary slowly in relation to the rotation of thewheel. Ideally (maximum passband), the number of sensors has to be atleast equal to the number of quantities to be estimated. Oneimplementation of this approach consists in providing the sensors 3 inan equally distributed fashion around the tire. Hence, in the event thatN sensors 3 have been fitted, the situation in which the sensors arecorrectly positioned occurs at least N times per revolution. FIG. 16presents this type of operation with three sensors, which arrive threetimes per revolution at the azimuths where the measurement is to becarried out (0°, 120° and 240°).

Lastly, it is possible to mix the approaches above.

Increasing the number of sensors makes it possible, in particular:

to increase the refresh frequency of the estimation of the forces, andtherefore the passband of the system

to increase the robustness with respect to rapid variations of thecomponents of the forces which are applied in the contact area.

It should be noted that it is possible to determine a plurality ofmodels which take the measurements at different azimuths as their input.Even with a single sensor, it is thus possible to obtain a plurality ofestimates during each wheel revolution.

FIG. 17 gives an example in which three sensors are used. Two transferfunctions are determined. The first uses measurements at 0°, 120° and240°, the second at 60°, 180° and 300°. When the sensors arrive at thedesired measurement positions, the transfer function can be applied. Bysuitably managing the sensors, it is even possible in this type ofarrangement to estimate the forces 6 times per wheel revolution. Theseestimates by a plurality of models may be averaged or compared in orderto increase the precision and reduce the noise in the estimation of theforces.

1. A method of determining at least one characteristic of a tireselected from: the x component, y component, and z component of aresultant of forces which are exerted by the road on the contact area ofa tire, the self-alignment torque generated by the tire, the camber andthe pressure, the method comprising the steps of measuring stresses inat least one bead of the tire at at least three fixed points in space,and deriving the characteristic from said at least one measurement. 2.The method according to claim 1, wherein the measurement of the stressesis performed in a rubber component whose Young's modulus is more than 5MPa at 10% strain.
 3. The method according to claim 1, the said threefixed points being selected such that: one of the points corresponds tothe azimuth of the center of the contact area or the azimuth of thepoint opposite to the contact area; and, two other points aresymmetrical with respect to a vertical plane passing through the centerof the contact area.
 4. The method according to claim 3, in which, themeasurement azimuths being selected symmetrically with respect to theazimuth of the center of the contact area (180°+α and 180°−α°), with αnot equal to 0° or 180°, V₁ ¹ and V₂ ¹ being values measured at theseazimuths on a first bead and V₁ ² and V₂ ² being the values measured atthese azimuths on a second bead, an estimate of the component Fx isprovided by f_(x)(a₁V₁ ¹+a₂V₂ ¹+b₁V₁ ²+b₂V₂ ²), where a₁, a₂, b₁ and b₂are positive real coefficients and f_(x) is a monotonic continuousfunction.
 5. The method according to claim 3, in which, the measurementazimuths being selected symmetrically with respect to the azimuth of thecenter of the contact area (180°+α and 180°−α°), with α not equal to 0°or 180°, V₁ ¹ and V₂ ¹ being values measured at these azimuths on thefirst bead and V₁ ² and V₂ ² being values measured at these azimuths onthe second bead, an estimate of the component Fz is provided byf_(z)(c₁V₁ ¹−c₂V₂ ¹+d₁V₁ ²−d₂V₂ ²), where c₁, c₂, d₁ and d₂ are positivereal coefficients and f_(z) is a monotonic continuous function.
 6. Themethod according to claim 3, in which, the measurement azimuths beingselected symmetrically with respect to the azimuth of the center of thecontact area (180°+α and 180°−α°), with α not equal to 0° or 180°, V₁ ¹and V₂ ¹ being values measured at these azimuths on the first bead andV₁ ² and V₂ ² being values measured at these azimuths on the secondbead, an estimate of the self-alignment torque N is provided byf_(n)(e₁V₁ ¹+e₂V₂ ¹−f₁V₁ ²−f₂V₂ ²), where e₁, e₂, f₁ and f₂ are positivereal coefficients and f_(n) is a monotonic continuous function.
 7. Themethod according to claim 3, in which, the measurement azimuths beingselected symmetrically with respect to the azimuth of the center of thecontact area (180°+α and 180°−α°), with α not equal to 0° or 180°, V₁ ¹and V₂ ¹ being values measured at these azimuths on the first bead andV₁ ² and V₂ ² being values measured at these azimuths on the secondbead, an estimate of the component Fy is provided by f_(y)(g₁V₁ ¹−g₂V₂¹−h₁V₁ ²+h₂V₂ ²), where g₁, g₂, h₁ and h₂ are positive real coefficientsand f_(y) is a monotonic continuous function.
 8. The method according toclaim 3, in which, the measurement azimuths being selected symmetricallywith respect to the azimuth of the center of the contact area (180°+αand 180°−α°), with α not equal to 0° or 180°, and V₁ and V₂ being valuesmeasured at these other azimuths, an estimate of Fz is provided byf_(z)(r₂V₂−r₁V₁), where r₁ and r₂ are positive real coefficients andf_(z) is a monotonic continuous function.
 9. The method according toclaim 3, in which, one of the azimuths corresponding to the middle ofthe contact area (azimuth 180°) and V_(c) being a value measured at thisazimuth, the other measurement azimuths being selected symmetricallywith respect to the azimuth of the center of the contact area (180°+αand 180°−α°), with α not equal to 0° or 180°, and V₁ and V₂ being valuesmeasured at these other azimuths, an estimate of Fy is provided byf_(y)(s_(c)V_(c)−(s₁V₁+s₂V₂)), where s₁, s₂ and s_(c) are positive realcoefficients and f_(y) is a monotonic continuous function.
 10. Themethod according to claim 3, in which, one of the azimuths correspondingto the middle of the contact area (azimuth 180°) and V_(c) being a valuemeasured at this azimuth, the other measurement azimuths being selectedsymmetrically with respect to the azimuth of the center of the contactarea (180°+α and 180°−α°), with α not equal to 0° or 180°, and V₁ and V₂being values measured at these other azimuths, an estimate of Fx isprovided by f_(x)(u_(c)V_(c)+u₁V₁+u₂V₂), where u₁, u₂ and u_(c) arepositive real coefficients and f_(x) is a monotonic continuous function.11. The method according to claim 1, wherein to estimate camber angle,the method comprises determining a difference in stresses being exertedin each of the beads on the basis of the measurements of stresses in thebeads.
 12. The method according to claim 1, wherein, to estimatepressure a contribution due to the pneumatic behaviour separate from acontribution due to structural behaviour is determined on the basis ofthe measurements of stresses in the beads.
 13. A method of determiningat least one characteristic of a tire selected from: the x component, ycomponent and z component of a resultant of forces which are exerted bythe road on the contact area of the tire, the self-alignment torquegenerated by the tire, the camber, and the pressure, the methodcomprising the steps of: determining measurement azimuths and collectingvalues for circumferential shear stress in a bead on at least one sideof the tire while soliciting varied stresses on the tire, which stressesare selected to cover the full range in which evaluation of the at leastone selected characteristic will be permitted in normal use, thesolicited stresses selected to create all the couplings expected duringnormal use, reading measured values for circumferential shear stress inthe bead and reading values of the at least one characteristicassociated with the measured values to form a training base, the valuesof the at least one characteristic being obtained through measurementmeans different from the means for the measured values, determiningcoefficients of a transfer function for establishing a link between themeasured values and the values of the at least one selectedcharacteristic on the basis of the training base, and, testing thetransfer functions by making and comparing estimates of the at least oneselected characteristic with the values obtained by differentmeasurement means.
 14. The method of determination according to claim13, in which the transfer function is a network having one layer ofhidden neurons and one layer of output neurons.